The invention relates to an organic electroluminescent device and in particular to an organic electroluminescent device with a capacitor overlying a transistor and method for fabricating the same.
Current electronic display products include, for example, cathode ray tubes (CRTs), liquid crystal displays (LCDs), plasma display panels (PDPs), and organic light emitting diodes (OLEDs). Among flat panel displays, OLEDs have the following advantages: light emission, high luminous efficiency, wide viewing angle, fast response time, high reliability, full color, low driving voltage, low power consumption, and simple fabrication. In a conventional passive organic luminescent device, the resolution is low and suitable only for a small-area and low-resolution display device. Conversely, an active organic luminescent device using thin film transistors (TFTs) in an active-addressing scheme has high resolution and low power consumption.
FIG. 1 is a schematic diagram of a pixel of a conventional active matrix OEL device. The pixel comprises a switch TFT 107, a drive TFT 109, a capacitor 111, and an organic light-emitting diode (OLED) 113. Gate, source, and drain electrodes of the switch TFT 107 are electrically connected to a scan line 101, a data line 103, and the capacitor 111, respectively. Gate, source, and drain electrodes of the drive TFT 109 are electrically connected to the drain electrode of the switch TFT 107, a Vdd line 105, and the OLED 113, respectively. When the scan line 101 is driven, the switch TFT 107 is switched on and the signal from the data line 103 is input into the capacitor 111. After the capacitor 111 is charged, the Vdd line 105 supplies a current to drive the OLED 113 according to the I-V characteristic of the drive TFT 109 and the voltage level of the capacitor 111.
FIG. 2 is a cross-section of a pixel structure of a conventional OEL device. In this structure, a switch TFT 107 is disposed on a substrate 100, comprising an active layer 102 with source and drain regions 102a and 102b, a gate dielectric layer 104, a gate electrode 106, a source electrode 110a, and a drain electrode 110b. An interlayer dielectric (ILD) layer 108 is disposed on the gate dielectric layer 104 and covers the gate electrode 106. A capacitor 111 is disposed on the substrate 100 outside of the switch TFT 107, comprising a bottom palate 110b (the drain electrode of the switch TFT 107), a passivation layer 112 serving as a capacitor dielectric layer, and a top electrode 114 electrically connected to a Vdd line 105. An insulating (passivation) layer 116 is disposed on the capacitor dielectric layer 112 and covers the top electrode 114. An OLED 113 is disposed on the substrate 100 outside of the TFT 107 and the capacitor 111, comprising a bottom electrode (not shown), an electroluminescent medium layer 118, and top electrode 120. Such a capacitor 111 arrangement occupies a portion of active area of the OLED 113, reducing the aperture ratio of the OEL device. In order to increase device resolution, the pixel size must be reduced, thus further increasing the occupied area of the capacitor. As a result, the aperture ratio of the OEL device is further reduced, such that the lifetime of the OEL device is reduced.